15.5 Turbine Engine Trimming & Post-Installation Procedures

15.5 Turbine Engine Trimming & Post-Installation Procedures

Quick Answer: Trimming adjusts the fuel control "to trim the engine to obtain maximum thrust output of the engine when desired," and FAA-H-8083-32B names four triggers: "The engine must be retrimmed after a fuel control unit is replaced, the engine does not develop maximum thrust, engine change, or excessive throttle stagger." The procedure begins by "obtaining the ambient air temperature and the field barometric pressure (not sea level) immediately preceding the trimming of the engine," from which "the desired turbine discharge pressure or EPR reading is computed from charts published in the maintenance manual." The engine is run "at full throttle (or at the part power control trim stop)" until stabilized — "five minutes is the usual recommended stabilization period" — with the compressor bleed valves verified closed and uncorrected accessory bleeds turned off. "Engines should never be trimmed if icing conditions exist," and "most electronic control fuel control systems do not require trimming or mechanical adjustments."

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What Trimming Actually Is

Section 13.3 covered the fuel control unit as the device that schedules fuel mass flow against power lever angle, spool speed, inlet temperature, and compressor discharge pressure. Trimming sets the reference point of that schedule.

"The fuel control unit of the typical turbofan on older aircraft can be a hydromechanical device that schedules the quantity of fuel flowing to the engine so that the desired amount of thrust can be obtained. The amount of thrust is dictated by the position of the power lever in the flight deck and the particular operation of the engine. Thus, the thrust of the engine and the consequent rpm of its turbine are scheduled by fuel flow."

The practical consequence: a hydromechanical fuel control unit built to specification still has to be matched to the specific engine it is bolted to and to the specific day it is being run on. Two nominally identical engines will not produce the same EPR at the same fuel-control setting, and one engine will not produce the same EPR on a hot day as on a cold one. Trimming resolves both.

The Four Triggers

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Establishing the Target: Ambient Conditions First

This is the step candidates skip and examiners ask about.

"In general, the procedure consists of obtaining the ambient air temperature and the field barometric pressure (not sea level) immediately preceding the trimming of the engine. Care must be taken to obtain a true temperature reading comparable to that of the air that enters the engine. Using these readings, the desired turbine discharge pressure or EPR (engine pressure ratio) reading is computed from charts published in the maintenance manual."

Three things are doing real work in that paragraph:

1. Field barometric pressure, not sea-level pressure. The altimeter setting broadcast by a tower is corrected to sea level. The engine breathes the actual station pressure. Using the sea-level value on a field several thousand feet up produces a target that the engine cannot meet, and the technician then trims fuel flow upward chasing a number that was wrong to begin with.
2. A true inlet-air temperature. A thermometer lying on hot asphalt in the sun is not reading the air the engine will ingest. The handbook's phrasing — "comparable to that of the air that enters the engine" — is the standard.
3. Immediately preceding the run. Conditions drift. A target computed an hour earlier is a target for a different day.

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Running the Trim

Stabilization and Configuration

"The engine is operated at full throttle (or at the part power control trim stop) for a sufficient period of time to ensure that it has completely stabilized. Five minutes is the usual recommended stabilization period."

Before reading anything, two configuration checks:

• "A check should be made to ensure that the compressor air-bleed valves have fully closed." Section 3.3 explained why those valves open at low rpm; if one is hung partially open at trim power, the engine is dumping compressed air overboard and will read low on EPR no matter how much fuel is added.
• "...and that all accessory drive air bleed for which the trim curve has not been corrected (such as a cabin air-conditioning unit) has been turned off." Bleed extraction that the chart does not account for is indistinguishable from a weak engine.

Making the Adjustment

"When the engine has stabilized, a comparison is made of the observed and the computed turbine discharge pressure Pt7 (or EPR) to determine the approximate amount of trimming required. If a trim is necessary, the engine fuel control is then adjusted to obtain the target turbine discharge pressure Pt7 or EPR on the gauge."

On a hydromechanical control, the physical adjustment is made with a screw: "If a hydromechanical fuel control is not within limits, turn the INC. MAX screw about one-eighth turn in the appropriate direction. Repeat, if necessary, until the desired value is attained." The tooling detail is worth noting because the two adjustments use different drivers — the maximum adjustment takes a 0.1875 hex socket and the rpm adjustment a 0.125 hex socket on the control the handbook illustrates.

Record what happens next. "Immediately following the fuel control adjustment, the tachometer reading is observed and recorded. Fuel flow and exhaust gas temperature readings should also be taken." Those three numbers, taken at the trimmed condition, are how the engine's health is judged. An engine that reaches target EPR only at abnormally high rpm, high fuel flow, or high EGT has reached it by working harder, which is a deterioration finding rather than a successful trim.

Then set idle. "After trimming the engine, the idle rpm can be adjusted. The idle rpm is adjusted by turning the INC. IDLE screw an eighth of a turn at a time, allowing sufficient time for the rpm to stabilize between adjustments. Retard the power lever to idle and recheck the idle rpm." Note the order: maximum first, then idle, and one eighth of a turn at a time with time to stabilize in both cases.

Wind, Moisture, and Ice

The environmental restrictions are safety and accuracy rules together:

• "If wind velocity is a factor, the aircraft should be headed into the wind while trimming or checking the trim on an engine." And again: "To obtain the most accurate results, the aircraft should always be headed into the wind while the engine is being trimmed."
• "Since trimming accuracy decreases as windspeed and moisture content increase, the most accurate trimming is obtained under conditions of no wind and clear, moisture-free air."
• "Do not trim when there is a tailwind because hot exhaust gases may be reingested." Reingestion raises inlet temperature, which corrupts the reading and can drive the engine toward an overtemperature.
• "As a practical matter, the engine should never be trimmed when icing conditions exist because of the adverse effects on trimming accuracy."
• "With the aircraft headed into the wind, verify that the exhaust area is clear." Then "install an engine trim gauge to the T-fitting in the turbine discharge pressure line. Start the engine and allow it to stabilize for 5 minutes before attempting to adjust the fuel control."
• "Engine trimming should always be carried out under precisely controlled conditions... Precise control is necessary to ensure maintenance of a minimum thrust level upon which the aircraft performance is based. In addition, precise control of engine trimming contributes to better engine life in terms of both maximum time between overhaul and minimum out-of-commission time due to engine maintenance requirements."

That last sentence is the answer to "why does this matter": the aircraft's certificated takeoff performance assumes the engine makes its rated thrust. An under-trimmed engine invalidates the takeoff data; an over-trimmed one buys thrust with turbine life.

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Electronic Controls Change the Whole Task

"Most electronic control fuel control systems do not require trimming or mechanical adjustments. Changes to the EEC in the FADEC system is normally accomplished through software changes or changing the EEC."

What replaces the screwdriver is a trim check — a verification run rather than an adjustment. The handbook lists the parameters an electronic trim check can cover:

1. Minimum idle (percent N2)
2. Approach idle (percent N2)
3. 2.5 bleed open (percent N1)
4. 2.5 bleed closed (percent N1)
5. Takeoff engine pressure ratio (EPR)
6. 95 percent takeoff thrust (EPR)
7. 90 percent thrust change decal (EPR)

"The actual trim check would be done based on a temperature and pressure... For these given temperature and pressures, the target parameter values can be derived from a chart in the manufacturer's manual. The engine is run up, and these values are checked against the tolerances given in the manual."

The structure is identical to the hydromechanical case — ambient conditions in, chart target out, engine run, comparison made — but the outcome is pass or fail rather than pass or adjust. An electronic engine that fails its trim check is not adjusted; it is investigated.

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Procedures Required After Installing a Turbine Engine

The ACS lists "procedures required after the installation of a turbine engine" as its own knowledge element. It is the mirror image of the removal sequence in Section 15.1.

Connections and Ducting

• "The system of ducts for routing air to the engine varies with the type" of installation, and "the filters in the air induction system must be cleaned" before the engine is run.
• "The exhaust system should also be carefully connected."
• Metal tubing with threaded fittings, hoses, and low-pressure hose installations each have their own connection rules; the general requirement is that every line, duct, and harness disconnected at removal is reconnected, secured, and safetied, and that every protective cap and plug installed at removal is accounted for. A cap left in a line is a post-installation defect that only shows up as a system failure on the first run.
• "Electrical leads within the engine nacelle are usually passed" through designated firewall provisions, and "when connecting the starter, generator, or various other" electrical accessories, the polarity, terminal torque, and bonding requirements of Sections 11.3 and 11.4 apply.

Control Rigging

"All engine controls must be accurately adjusted to ensure" correct operation. The distinctive turbine item is the power lever cushion: the cable turnbuckles are loosened until the throttle control reaches its stop, then "adjust the cushion by turning the cable turnbuckles equally" so that the flight-deck lever has the specified spring-back at the stop. "On multiengine aircraft, the amount of cushion of all engine" controls is matched — unequal cushion is one source of the throttle stagger that is itself a trim trigger.

On computer-controlled engines the linkage story changes: "Most computer controlled engines have an electronic" interface rather than a long cable run to the fuel control, so the rigging task moves to resolver or sensor alignment specified by the manufacturer.

Pre-Oiling and the First Run

"Before the new engine is flight tested, it must undergo" pre-oiling "to prevent failure of the engine bearings during the initial" start. External pressure is applied until "oil flowing out of the engine indicates the completion of the" process. Section 15.3 covers the reciprocating procedure in detail; the principle is identical on a turbine — no dry starts on bearings that have never seen oil pressure.

Fuel system bleeding follows: "To purge the fuel system of air locks, and to aid in flushing" the system before the first light-off.

After the Ground Run

"After the engine has been ground operated, and again after" the first flight, a defined re-inspection is made. "Especially, check the oil system hose clamps for security" — hot-torque and re-check items exist because clamps and fittings relax after the first thermal cycle. The general post-run inspection is a leak check, a security check, and a repeat of the torque and safety-wire verification on everything that was disturbed.

Then Trim, Then Record

The sequence closes where this section opened: an engine change is one of the four conditions that requires a trim, so the newly installed engine is trimmed or trim-checked against the chart before it is released. Everything is then recorded under 14 CFR § 43.9 as Section 14.4 describes, and the trimmed values — EPR, rpm, fuel flow, and EGT at the trim condition — become the baseline against which the next engine condition monitoring sample is compared.

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Independent Prep Note

Independent FAA AMT Powerplant prep by OpenExamPrep. Not sponsored by or affiliated with the Federal Aviation Administration (FAA). Technical data compiled from FAA-H-8083-32B, FAA AC 43.13-1B, and 14 CFR Parts 43 and 65.